The invention relates to process control and, more particularly, to systems for analyzing processes to determine characteristics such as dead time, primary and secondary time constants and static gain.
Process control refers to a methodology for controlling the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. While process control is typically employed in the manufacturing sector, it also has application in service industries.
A process control unit, or "controller," typically operates by comparing values of a process characteristic--referred to as the controlled variable--with a target value to determine whether the process is operating within acceptable bounds. For example, in a process in which fluid flows at a constant rate from a tank that is continuously filled to constant volume, a controller monitors the liquid level and, if necessary to prevent the tank from running dry or overflowing, adjusts an inlet valve to increase or restrict inflow to the tank.
In order to function properly, a controller must be adjusted to accommodate charcteristics of the specific process it will control. This requires identifying process parameters such as the primary time constant (which reflects the rate at which the process responds to changes in input), gain (which reflects the magnitude of response), and so forth.
Prior art techniques for identifying those parameters involve applying a single step to the process, monitoring the process response and, from that calculating the requisite process parameters.
For example, in a text previously authored by him, the inventor hereof suggests the following procedure for determining process dead time:
1. Place the process controller in manual mode and apply a single step to the process. PA1 2. Monitor the resultant change in output of the process. PA1 3. Graphically, or otherwise, determine the point of intersection between (a) the line defining process output prior to application of the step pulse, and (b) the tangent of maximum slope of the process response curve. PA1 (i) generating a signal, t.sub.1, representing a first time interval as a function of the mathematical expression EQU t.sub.1 =NB*.tau..sub.1 /.vertline..delta.m.vertline. PA1 (ii) generating a signal, t.sub.2, representing a second time interval having, initially, a value substantially equal to that of the first time interval, t.sub.1 ; PA1 (iii) iteratively regenerating the signal, t.sub.2, until its value no longer changes significantly between iterations; wherein, such regeneration is in accord with the mathematical expression EQU t.sub.2 =t.sub.1 +.tau..sub.2 *(1-e.sup.-t.sbsp.2.sup./.tau..sbsp.2); PA1 where .tau..sub.2 is a secondary time constant of the process as determined in accord with other aspects of the invention, as described below; and PA1 (iv) estimating the dead time .tau..sub.d as a function of the mathematical expression EQU .tau..sub.d =.tau..sub.a -t.sub.2 PA1 where t.sub.2 is a final value of that interval, as determined in step (iii), above.
The point of intersection identified in step 3 is taken as the dead time.
This and related techniques for determining process characteristics by monitoring response to a single step are generally quite effective. Nevertheless, an object of this invention is to provide more accurate methods and apparatus for analyzing process characteristics.
More particularly, an object of this invention is to provide a method and apparatus for determining process characteristics such as primary and secondary time constants, dead time, and steady state gain, among others, as effectively and accurately as possible.